The present invention is in the field of article surveillance techniques and relates to a magnetic marker for use in an electronic article surveillance system (EAS).
Magnetic markers are widely used in EAS systems, due to their property to provide a unique non-linear response to an interrogating magnetic field created in a surveillance zone. The most popularly used markers utilize a magnetic element made of soft magnetic amorphous alloy ribbons, which is typically shaped like an elongated strip. A marker of this kind is disclosed, for example, in U.S. Pat. No. 4,484,184. This strip-like marker usually is of several centimeters in length and a few millimeters (or even less then a millimeter) in width.
It is a common goal of marker designing techniques to decrease the marker dimensions and to enhance the uniqueness of its response. Additionally, it is desirable to increase the marker flexibility so as to enable its attachment to various flexible and flat articles like clothes, footwear, etc. in a concealed manner. For these purposes, a magnetic element in the form of a thin wire is preferable over that of a strip.
U.S. Pat. No. 5,801,630 discloses a method for preparing a magnetic material with a highly specific magnetic signature, namely with a magnetic hysteresis loop having large Barkhausen discontinuity at low coercivity values, and a marker utilizing a magnetic element made of this material. The material is prepared from a negative-magnetostrictive metal alloy by casting an amorphous metal wire, processing the wire to form longitudinal compressive stress in the wire, and annealing the processed wire to relieve some of the longitudinal compressive stress. However, a relatively large diameter of the so-obtained wire (approximately 50 xcexcm) impedes its use in EAS applications. Additionally, a complicated multi-stage process is used in the manufacture of this wire. Furthermore, amorphous wire brittleness unavoidably occurs, due to the wire-annealing process. Such brittleness will prevent the use of the wire in flexible markers.
A technique for manufacturing microwires known as Taylor-wire method enables to produce microwires having very small diameters ranging from one micrometer to several tens micrometers by a single-stage process consisting of a direct cast of a material from melt. Microwires produced by this technique may be made from a variety of magnetic and non-magnetic alloys and pure metals. This technique is disclosed, for example, in the article xe2x80x9cThe Preparation, Properties and Applications of Some Glass Coated Metal Filaments Prepared by the Taylor-Wire Processxe2x80x9d, W. Donald et al., Journal of Materials Science, 31, 1996, pp. 1139-1148.
The most important feature of the Taylor-wire process is that it enables to produce metals and alloys in the form of a glass-coated microwire in a single operation, thus offering an intrinsically inexpensive way for the microwire manufacture.
A technique of manufacturing magnetic glass-coated microwires with an amorphous metal structure is described, for example, in the article of xe2x80x9cMagnetic Properties of Amorphous Fe-P Alloys Containing Ga, Ge and Asxe2x80x9d, H. Wiesner and J. Schneider, Phys. Stat. Sol. (a) 26, 71 (1974).
The properties of amorphous magnetic glass-coated microwires are described in the article xe2x80x9cHigh Frequency Properties of Glass-Coated Microwiresxe2x80x9d, A. N. Antonenko et al, Journal of Applied Physics, vol. 83, pp. 6587-6589. The microwires cast from alloys with small negative magnetostriction demonstrate flat hysteresis loops with zero coercivity and excellent high frequency properties. The microwires cast from alloys with positive magnetostriction are characterized by ideal square hysteresis loops corresponding to their single-domain structure.
There is accordingly a need in the art to facilitate the article surveillance by providing a novel magnetic marker to be used in EAS system.
It is a major feature of the present invention to provide such a marker that has minimum dimensions, while maintaining the necessary level of response to an interrogating magnetic field.
It is a further feature of the present invention that the marker has highly unique response characteristics.
It is a still further feature of the present invention that the marker is extremely flexible, and can therefore be introduced to articles made of fabrics and having a complex shape.
The main idea of the present invention is based on the use of amorphous metal glass-coated magnetic microwires with substantially zero magnetostriction, very low coercivity (substantially less than 10 A/m) and high permeability (substantially higher than 20000) to form a magnetic element of a marker. The present invention takes advantage of the use of the known Tailor-wire method for manufacturing these amorphous glass-coated magnetic microwires from materials enabling to obtain the zero magnetostriction.
Although amorphous magnetic glass-coated microwires and their manufacture have been known for a long time, no attempts were made for using them in magnetic elements of EAS markers. These amorphous magnetic glass-coated microwires, however, have good mechanical strength, flexibility, and corrosion resistance, and can therefore be easily incorporated in paper, plastic, fabrics and other article materials.
There is thus provided according to one aspect of the present invention, a magnetic marker for use in electronic article surveillance (EAS) system, the marker comprising a magnetic element formed by at least one microwire piece made of an amorphous metal-containing material coated with glass, the microwire piece having substantially zero magnetostriction, coercivity substantially less than 10 A/m and permeability substantially higher than 20000.
Preferably, the microwire piece is manufactured by a single-stage process of direct cast from melt (i.e., Tailor-wire method). The microwire (its metal core) has a desirably small diameter, (e.g., several micrometers) substantially hot exceeding 30 xcexcm. The properties of the microwire piece are controlled by varying the metal-containing material composition and the glass-to-metal diameter ratio.
The microwire piece comprises a core, made of the metal-containing material, and the glass coating. The metal core and the glass coating may be either in continuous contact or may have only several spatially separated points of contact.
Preferably, the metal containing material is a cobalt-base alloy. For example Coxe2x80x94Fexe2x80x94Sixe2x80x94B alloy (e.g., containing 77.5% Co, 4.5% Fe, 12% Si, and 6% B by atomic percentage), Coxe2x80x94Fexe2x80x94Sixe2x80x94Bxe2x80x94Cr alloy (e.g., containing 68.7% Co, 3.8% Fe, 12.3% Si, 11.4% B, and 3.8% Cr by atomic percentage), or Coxe2x80x94Fexe2x80x94Sixe2x80x94Bxe2x80x94Crxe2x80x94Mo alloy (e.g., containing 68.6% Co, 4.2% Fe, 12.6% Si, 11% B, 3.52% Cr and 0.08% Mo by atomic percentage) may be used. The microwire piece made of the Coxe2x80x94Fexe2x80x94Sixe2x80x94Bxe2x80x94Crxe2x80x94Mo alloy shows less sensitivity to external mechanical tensions, due to the fact that in this microwire the metal core and glass coating are physically attached to each other only in several spatially separated points of contact, rather than being in continuous contact.
According to one embodiment of the invention, the marker is in the form of a strip, formed by several parallel microwire pieces enclosed between substrate and cover layers. The substrate and cover layers are, preferably, manufactured by a co-extrusion process.
According to another embodiment of the invention, the magnetic element is in the form of a plurality of the microwire pieces twisted in a thread, and optionally comprises auxiliary non-magnetic reinforcement fibers. Preferably, the thread is soaked with an elastic binder.
According to yet another embodiment of the invention, the magnetic element is formed by a plurality of the microwire pieces aligned in a bundle and assembled in a thread by winding non-magnetic auxiliary fibers. The auxiliary fibers may either partly on entirely cover the outer surface of the bundle.
According to another aspect of the present invention, there is provided an electronic article surveillance system utilizing a market mounted within an article to be detected by the system when entering an interrogation zone, the system comprising a frequency generator coupled to a coil for producing an alternating magnetic field within said interrogation zone, a magnetic field receiving coil, a signal processing unit and an alarm device, wherein said marker comprises a magnetic element formed by at least one microwire piece made of an amorphous metal-containing material coated with glass, the microwire piece having substantially zero magnetostriction, coercivity substantially less than 10 A/m and permeability substantially higher than 20000.